Update custom config note (#5988)

install/tpl/debian6_dovecot2.conf.master

-# Do not change this file, as changes will be overwritten by any ISPConfig update. Put your custom settings in /etc/dovecot/conf.d/99-custom-config.conf
+# Do not change this file, as changes will be overwritten by any ISPConfig update.
+# Put your custom settings in /usr/local/ispconfig/server/conf-custom/install/dovecot_custom.conf.master.
+# To start using those changes, do a force upgrade and let it reconfigure your services. (ispconfig_update.sh --force)
 listen = *,[::]
 protocols = imap pop3
 auth_mechanisms = plain login

install/tpl/debian_dovecot2.conf.master

-# Do not change this file, as changes will be overwritten by any ISPConfig update. Put your custom settings in /etc/dovecot/conf.d/99-custom-config.conf
+# Do not change this file, as changes will be overwritten by any ISPConfig update.
+# Put your custom settings in /usr/local/ispconfig/server/conf-custom/install/dovecot_custom.conf.master.
+# To start using those changes, do a force upgrade and let it reconfigure your services. (ispconfig_update.sh --force)
 listen = *,[::]
 protocols = imap pop3
 auth_mechanisms = plain login

install/tpl/fedora_dovecot2.conf.master

-# Do not change this file, as changes will be overwritten by any ISPConfig update. Put your custom settings in /etc/dovecot/conf.d/99-custom-config.conf
+# Do not change this file, as changes will be overwritten by any ISPConfig update.
+# Put your custom settings in /usr/local/ispconfig/server/conf-custom/install/dovecot_custom.conf.master.
+# To start using those changes, do a force upgrade and let it reconfigure your services. (ispconfig_update.sh --force)
 listen = *,[::]
 protocols = imap pop3
 auth_mechanisms = plain login